The present invention relates to signal equalizers. In particular, the present invention relates to a signal equalizer that compensates for the uneven response, across a frequency range, of devices inserted in the path of a radio frequency communication network.
In the radio frequency (RF) communication field and in particular, broadband technologies such as Cable TV (CATV), a passive component called a splitter is used to distribute a signal to multiple outlets. Conversely, a passive component called a combiner is used to join multiple signals to a common outlet or port. In the CATV environment, the signals are broadband in nature in that they typically occupy a spectrum of 5 MHz through 1000 MHz.
It is desired that the frequency response of passive components not only have a minimum of insertion loss, but also exhibit a loss that is independent of frequency. A typical 8-way splitter/combiner, for example, exhibits an insertion loss of approximately 10 dB at 5 MHz and 12.6 dB at 1000 MHz. The insertion loss yields a downward tilt in frequency response of approximately 2.6 dB. The downward tilt is undesirable, and in conjunction with other elements in the distribution network, creates a severe imbalance in the system end-to-end frequency response flatness.
In addition, coaxial cable contributes to a substantial amount of frequency response tilt proportional to the square root of frequency (with the maximum insertion loss being at 1000 MHz). In the past, coaxial cable equalizer circuits have been developed to compensate for the frequency response tilt caused by the coaxial cable itself. The coaxial cable equalizer circuits are typically implemented as plug-in modules that are associated with system amplifiers and that have a frequency response curve designed to match the coaxial cable frequency response tilt. However, the coaxial cable equalizers are designed under the assumption that the coaxial cable not only has an exact characteristic impedance of 75 ohms, but is also terminated in exactly 75 ohms. As a result, the coaxial cable equalizers do not compensate for the influence of impedance mismatches in the coaxial cable or the cable""s terminations.
Passive devices, including those noted above, have a frequency response tilt that is generally directly proportional to frequency. Thus, the passive device frequency response tilt does not track that of the coaxial cable itself. As a result, the prior coaxial cable equalizers were unable to properly compensate for the frequency response tilt arising from passive devices used with the coaxial cable.
A need has long existed in the industry for an RF equalizer that addresses the problems noted above and others previously experienced.
An improved RF equalizer is arrived at by using a voltage divider L-attenuator (referred to below simply as a xe2x80x9cvoltage dividerxe2x80x9d). The RF equalizer is preferably incorporated into a network device such as a directional coupler, splitter, combiner, or the like. However, the RF equalizer may be implemented as a stand-alone add-on to existing network devices whose S-parameter characteristics are known.
The RF equalizer provides a frequency variable compensation attenuation and generally includes a series impedance and an impedance to ground (i.e., a shunt impedance). The primary components of the series and shunt impedances are resistances. Preferably, the series impedance also includes a parallel combination of a bypass circuit (e.g., a series inductance and capacitance) and an augmenting circuit (e.g., a capacitor), while the shunt impedance is formed from a series RLC circuit.
The bypass circuit, when implemented as a series LC resonant circuit, is preferably tuned to present a minimal impedance (i.e., a short or bypass) around the series resistance at the highest frequency of interest (e.g., 1000 MHz). At frequencies below resonance, the bypass circuit provides a net capacitive reactance which is augmented by the fixed capacitor in the augmenting circuit. In other words, the augmenting circuit presents an additional capacitance in parallel with the net capacitive reactance of the bypass circuit. The series impedance decreases with increasing frequency (and is virtually shorted out at the highest frequency of interest), thereby providing a frequency variable compensation attenuation with properties described in more detail below.
While the frequency variable compensation attenuation may vary depending on the application, it may, as one example, follow a linear relationship between a first frequency and a second frequency. In the CATV industry, the first frequency may be approximately 5 MHz and the second frequency may be 1000 MHz. Continuing with the example, the linear relationship, at the first frequency, may provide a low frequency attenuation approximately matching a difference in frequency response of a precharacterized network device between the first frequency and the second frequency. In other words, assuming, as an example, that an 8-way splitter has a frequency response 2.6 dB greater at the first frequency than the second frequency, then the low frequency attenuation is preferably 2.6 dB.
Other implementations, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims.